1. Field of the Invention
The field of this invention is the drying of wet wood waste having a wide range of particle sizes, such as hog fuel. More particularly, the invention relates to achieving a uniform moisture content without overdrying fines portions of the waste.
2. Description of the Prior Art
Wood wastes are widely used in the forest products industry as boiler fuel to produce steam. Such wastes, commonly called "hog" or "hogged" fuel, are generally a mixture of, for example, bark, wood chips, planer shavings, sawdust and forest residues, including some sand and rocks. Particle size diameters may range from 0.01 inches for sanderdust to several inches for bark. The average particle size for U.S Pacific Northwest hog fuel is 3/4 inch while that of the Southeast averages 3/8 inch. Fine particles, those less than 1/8 inch diameter, comprise about 15-50% of hog fuel.
The moisture content of hog fuel varies widely depending upon such factors as species, weather, production methods and storage patterns. The moisture content for commercial hog fuel may range from 30% to 65% by weight but is normally fired to the boiler at about 45%-55%.
The moisture in hog fuel significantly reduces its value as boiler fuel. At 50% moisture, approximately 12% of the energy of the fuel itself is required to vaporize the moisture. The high flow rate of the water vapor though the boiler decreases the maximum temperature of combustion gases, degrading heat transfer to the steaming tubes of the boiler. Further, the large volume of water vapor in the boiler exhaust produces a sizeable heat loss.
If hog fuel is dried from 50% to 30% moisture content before burning, boiler efficiency is increased 12% and steaming rate would concurrently increase 17%. An ancillary benefit of burning dried hog fuel is a reduction in particulates in the stack gas, due to more complete combustion of the carbon content of the wood. Also, the use of auxillary fuel such as oil, typically necessary to sustain combustion, may be reduced. Dry hog fuel also offers such significant performance advantages that alternative methods of heat recovery from hog fuel become more practical. For example, firing a fines portion of the wood fuel through a pulverized coal-type suspension burner or producing a fuel gas from the dried wood in a gasifier bed may be reasonably contemplated.
Hog fuel dryers are well known in the forest products industry. Some dryers use flue gas from the wood fired boiler to dry incoming wet hog fuel. Others use hot exhaust gases from some separate combustion device, while a few dryers use steam. Most installations are rotary or cascade-type dryers.
Rotary dryers tumble the hog fuel in a long horizontal cylinder while passing hot gases through the cylinder to perform the drying. The wet hog fuel and hot gases enter at the same end of the dryer. The hog fuel moves through the dryer due to the aerodynamic force of the hot gases and a slight downward tilt of the axis of the dryer. The finest particles of hog fuel are simply blown through the dryer by the hot gas. Larger particles may take from 5 minutes to 30 minutes to transit the dryer.
Hog fuel absorbs moisture easily because of its open porous structure. At 50% moisture content, relatively little surface moisture is evident. As the moisture content increases to 60-65%, surface moisture increases greatly and the hog fuel appears soaking wet. Dryers are typically designed to reduce the average moisture content of the hog fuel to 30-40% before firing in the boiler. If the moisture content is reduced below 30%, dusting occurs resulting in housekeeping problems and fire hazards.
To dry hog fuel to 30-40% moisture content, the moisture must diffuse through the pores structure of the fuel before it can evaporate from the surface. This diffusion rate controls the drying time of hog fuel. Large particles require substantially longer times to dry than small ones because of the difficulty of diffusing moisture to the surface. Drying hog fuel to a truly uniform moisture content is difficult because of the wide variation in particle size.
In a rotary dryer, transit time of the fuel through the dryer is set to achieve an overall average moisture content of, for example, 40%. However, in the typical rotary dryer product, the largest particles will contain substantially more moisture than 40% while the smaller ones will range from perhaps 5 to 15%. A major problem arises from drying the smaller particles to a low moisture content. Inlet hot gases to the dryer range from 450.degree. F. to 1000.degree. F. and the exit gases are usually over 200.degree. F. During the period that water is evaporating from the surface of a particle, it remains near the wet bulb temperature of the gas, 140.degree. F. to 160.degree. F. When the water has evaporated or nearly so, the particle begins to increase in temperature due to heat transfer from the hot gas. As the wood particles increase in temperature above 160.degree. F., they begin to release volatile hydrocarbons. These volatiles, when released to the atmosphere, are air pollutants commonly called "blue haze." Blue haze represents a serious air pollution limitation, substantially restricting the recovery of heat from hog fuel. Blue haze is particularly bothersome when drying wood particles finer than hog fuel, such as sawdust for use in the manufacture of particleboard. For particleboard manufacture, the desired moisture content of the product is 0% rather than the 30% desired for hog fuel and the hot drying gases are typically in the range of 1000.degree. F.
Rotary dryers have other disadvantages. Heat transfer between hot gases and hog fuel is limited because the fuel in the dryer spends the majority of its time laying in the flights of the drum and only a short time falling through the hot gases, where heat transfer principally occurs. Hence, to accomplish the necessary overall heat transfer, rotary dryers tend to be large and require substantial plantsite space.
Cascade dryers entrain and re-entrain the hog fuel in a high volocity upward flow of hot gases directed along the centerline of a vertical cylindrical vessel. Near the top of the cylinder, the hog fuel is directed toward the wall of the vessel while the gas escapes through an outlet at the top. The hog fuel falls downward along the wall and is re-entrained in the jet of hot gases entering at the bottom of the vessel. Dried fuel exits near the wall at a location away from the entrance. The average residence time for the hog fuel in the cascade dryer is two minutes. The smaller fine particles are blown immediately and directly out with the exhausting hot gases.
The cascade dryer overcomes the low heat transfer rate problem of the rotary dryer. Heat transfer rates are excellent at the high relative gas velocities and the hog fuel is exposed to these conditions for a significant portion of its transit time. Cascade dryers are significantly smaller than rotary dryers of equivalent capacity. However, the blue haze problem remains. In fact, the problem is exacerbated because of the high drying rates resulting from the high relative gas velocity and the repeated reintroduction of the drying particles into contact with high temperature inlet gases. In the short residence time of two minutes, the water content of larger particles has little chance to diffuse to the surface of the particle, regardless of how efficiently it is removed from the surface. Hence, in order to meet any specified average exit moisture condition, some particles tend to be overdried.
Fluid or fluidized bed dryers are well known for the high rate of heat transfer between the gas and the fluidized particles as well as between bed particulates and surfaces immersed in the bed. Heat transfer coefficients in fluid beds range to 40 BTU/Hr-Ft.sup.2 -.degree.F. while similar heat transfer coefficients for a surface exposed to a hot gas stream without the presence of a fluid bed would be perhaps 10 BTU/Hr-Ft.sup.2 -.degree.F. Heretofore, fluid bed dryers have principally been used for drying homogeneous finely-divided materials whose fluidization characteristics are well known or can be predicted with precision. Granular materials such as activated carbon, coal and plastic beads are routinely dried in fluid bed dryers.
The drying of particulate coal in a fluidized bed is well known, employing, most often, hot combustion gases to fluidize the bed and provide the enthalpy necessary to dry the coal. U.S. Pat. No. 3,755,912 to Hamada, et al., describes a process wherein hot off-gases from a coking oven are used to fluidize and dry a bed of coal. U.S. Pat. No. 3,190,627 to Goins reveals a fluidized bed dryer using a plurality of gas-fired burners to supply hot gas to the fluid bed.
Several processes utilize the combustion of coal to provide the necessary heat for the fluid bed dryer. U.S. Pat. No. 3,896,557 to Seitzer, et al., provides for the collection of coal fines above the fluidizing drying bed and the burning of these fines in a separate combustion chamber to produce products of combustion to fluidize and heat the drying bed.
Jukolla in U.S. Pat. No. 2,638,684 describes drying coal in two fluidized beds arranged in a single vessel. A fines portion of coal is separated from the upper fluidized bed dryer coal product and injected into a lower combustion bed. The lower bed combustion gases provide the drying heat for the coal at sufficient velocity to fluidize the inert solids drying bed and substantially dry and entrain all of the coal fed to the drying bed. The dried, entrained coal is swept from the bed and passes through a series of cyclones which produces a dried coal product and the fines portion for combustion. The Jukkola process requires the use of inert solids fluid beds if coal in excess of 7% moisture is to be dried under stable production conditions. The process would not be suitable for drying hog fuel having a wide particle size range and sensitivity to overdrying.
Difficult waste materials such as sewage and refinery sludges are dried in fluid beds. However, as in Jukkola, these fluid beds are essentially sand beds where the waste material comprises only a small portion of the bed material and does not significantly after the fluidization characteristics of the inert sand. Fitch, U.S. Pat. No. 4,159,682 teaches drying of sludges in such a sand fluid bed using an inflow of hot sand from a fluid bed combustor to supply the heat. The cooled sand mixed with the dried sludge is transported back to the fluid bed for combustion.
In comparison with coal drying, the drying of wood waste and the like to fluidized beds is a relatively recent art. The nonuniformity of the typical wet wood to be dried has always been the principal problem to be overcome.
Voelskow, U.S. Pat. No. 3,721,014, teaches drying wood particles for particleboard by using two aerodynamic separators employing hot gases to segregate a fine fraction from a coarse fraction. Voelskow recognized the problem of overdrying the fines fraction while attmpting to dry the coarse fraction. Voelskow solved the problem by separating the fractions and drying them separately.
Spurrell in U.S. Pat. No. 4,235,174 teaches the use of a fluid bed combustor burning an oversize waste wood fraction to supply hot gases to a conventional rotary dryer to dry the balance of the hog fuel pile. Output of the dryer is screened into fine and coarse fractions. The fine fraction is burned in a wood-fired boiler in a suspension, pulverized coal type burner while the coarse fraction is burned on the gate. Spurrell does not suggest substituting a fluid bed dryer for drying hog fuel in place of the conventional rotary dryer.
Ide, et al., in French Patent Application No. 76 31487 describes a fluid bed dryer for drying and separating degradable organics for fertilizer composting from biologically inert granular material. The fluid bed dryer has a distributor plate which causes fluidized drying material to move in a spiral path from the center outward. A mechanical arm rotates in the fluid bed to break up lumps of material and to promote smooth fluidization of difficult materials.